The objective of the proposed study is to determine whether conservation of liquid quantity with logical necessity, as measured by a set of violation tasks, requires formal operational reasoning. Based on Piagetian theory it was shown that logical necessity is a formal operational notion. Theoretically, it was shown that conservation of liquid quantity with logical necessity is equivalent to conservation of occupied volume of the liquid. It was hypothesized that: (1) there will be no significant difference in performance on violation tasks and conservation of volume tasks for Level I and Level II; and (2) conservation of mass and conservation of interior volume will precede conservation of weight and, conservation of weight will precede conservation of volume for Level I and Level II and conservation of liquid quantity with logical necessity. McNemar's test for equality of correlated proportions will be used to test hypothesis 1. Heiner's statistical technique for testing hypothesized developmental structures will be used to test hypothesis 2. A sample of 144 subjects, 12 males and 12 females per grade, grades 3 to 8, will be selected to test the hypotheses.